HL IB Physics Greenhouse Effect PDF

Head to www.savemyexams.com for more awesome resources HL IB Physics Your notes Greenhouse Effect Contents Albedo & Emissivity The Solar Constant Greenhouse Gases The Greenhouse Effect Energy Balance...

Head to www.savemyexams.com for more awesome resources HL IB Physics Your notes Greenhouse Effect Contents Albedo & Emissivity The Solar Constant Greenhouse Gases The Greenhouse Effect Energy Balance Problems Page 1 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Albedo & Emissivity Your notes Emissivity Stars are good approximations to a black body, whereas planets are not This can be quantified using the emissivity Emissivity, e, is defined as The the ratio of the power radiated per unit area by a surface compared to that of a black body at the same temperature It can be calculated using the equation power radiated by an object e= power emitted by a black body Calculations of the emissivity assume that the black body: Is at the same temperature as the object Has the same dimensions as the object For a perfect black body, emissivity is equal to 1 When using the Stefan-Boltzmann law for an object which is not a black body, the equation becomes: P = eσAT4 Where: P = total power emitted by the object (W) e = emissivity of the object σ = the Stefan-Boltzmann constant A = total surface area of the object black body (m2) T = absolute temperature of the body (K) Examiner Tip You will be expected to remember that a perfect black body has an emissivity of 1 - this information is not included in the data booklet! Page 2 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Albedo Albedo, a, is defined as Your notes The ratio of the total scattered power to the total incident power of radiation that is reflected by a given surface It can be calculated using the equation total scattered power a= total incident power More specifically, the albedo of a planet is defined as The ratio between the total scattered, or reflected, radiation and the total incident radiation of that planet Earth’s albedo is generally taken to be 0.3, which means 30% of the Sun’s rays that reach the ground are reflected, or scattered, back into the atmosphere An albedo of 1 represents a surface that scatters all the incident radiation Earth’s albedo varies daily and depends on: Cloud formations and season – the thicker the cloud cover, the higher the degree of reflection Latitude Terrain – different materials reflect light to different degrees Incident angle of radiation It is useful to know the albedo of common materials: Fresh asphalt = 0.04 Bare soil = 0.17 Green grass = 0.25 Desert sand = 0.40 New concrete = 0.55 Ocean ice = 0.50 - 0.70 Fresh snow = 0.85 Albedo has no units because it is a ratio (or fraction) of power Page 3 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Worked example The average albedo of fresh snow is 0.85 energy absorbed by fresh snow Calculate the ratio energy reflected by fresh snow Answer: Step 1: Define albedo Albedo = the proportion of radiation that is reflected Therefore, the energy reflected by fresh snow = 0.85 Step 2: Identify the proportion of radiation that is absorbed If 85% of the radiation is reflected, we can assume that 15% is absorbed Therefore, the energy absorbed by fresh snow = 1 – 0.85 = 0.15 Step 3: Calculate the ratio energy absorbed by fresh snow 0. 15 = = 0. 18 energy reflected by fresh snow 0. 85 Page 4 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Solar Constant Your notes The Solar Constant Since life on Earth is entirely dependent on the Sun’s energy, it is useful to quantify how much of its energy reaches the top of the atmosphere This is known as the solar constant, S The solar constant is defined as: The intensity of the Sun's radiation arriving perpendicularly to the Earth's atmosphere when the Earth is at its mean distance from the Sun The average value of the solar constant is 1.36 × 103 W m−2 The value of solar constant varies year-round because: The Earth is in an elliptical orbit around the Sun, meaning at certain times of year the Earth is closer to the Sun, and at other times of year it is further away The Sun’s output varies by about 0.1% during its 11-year sunspot cycle Calculations of the solar constant assume that: This radiation is incident on a plane perpendicular to the Earth's surface The Earth is at its mean distance from the Sun The intensity of solar radiation received by different planets in the Solar System varies depending on distance from the Sun For example, the intensity of solar radiation incident on Venus' atmosphere is higher than Earth's because it is closer to the Sun Incoming Radiative Power The surface area of a planet, with radius r, equals the surface area of a sphere, 4πr2 A planet's radiative intensity covers a cross-sectional area of πr2 So the mean value of the radiative power or intensity is: ⎛⎜ πr 2 ⎞⎟ S S×⎜ ⎟= ⎝ 4πr 2 ⎠ 4 Page 5 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes The Sun emits 4 × 1026 J in one second. The mean distance of the Earth from the Sun is 1.5 × 1011 m. Using this data, calculate the solar constant. Answer: Step 1: List the known quantities Power output of Sun, P = 4 × 1026 W Distance between the Earth and Sun, r = 1.5 × 1011 m Step 2: Model the scenario using geometry As light leaves the surface of the Sun, it begins to spread out uniformly through a spherical shell The surface area of a sphere = 4πr2 The radius r of this sphere is equal to the distance between the Sun and the Earth Step 3: Write an equation to calculate the solar constant P Solar constant = 4πr 2 Page 6 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Step 4: Calculate the solar constant 4 × 1026 Your notes Solar constant = = 1415 W m−2 4π(1. 5 × 1011 ) Solar constant = 1.4 kW m–2 (2 s.f) Examiner Tip When defining the solar constant, it is important to say that the radiation arrives above the Earth's atmosphere and not at the Earth's surface. Page 7 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Greenhouse Gases Your notes The Main Greenhouse Gases The main greenhouse gases have both natural and human-generated origins In order of decreasing contributions, these are: Water vapour (H2O) - evaporation from the oceans / seas and plants Carbon dioxide (CO2) - volcanic eruptions, wildfires and respiration Methane (CH4) - emission from oceans and soils as part of decomposition, termites also emit methane Nitrous oxide (N2O) - soils and oceans When radiation from the Sun hits the Earth, it is radiated back from the Earth's surface as long-wave radiation A greenhouse gas is a gas that absorbs this re-radiated radiation, trapping it in the Earth's atmosphere so that it is not lost to space Greenhouse gases in the atmosphere have a similar effect to the glass in a greenhouse, hence the term greenhouse gas There are many greenhouse gases, and those that contribute most to the greenhouse effect are: Carbon dioxide (CO2) Water vapour (H2O) These have the most significant impact on the greenhouse effect There are other greenhouse gases which have a lesser effect, such as: Ozone (O2 and O3) Methane Nitrous oxides Examiner Tip You may have heard of a separate environmental concern, described as the 'hole in the ozone layer'; this is not something that you need to know about. Ozone is an atmospheric gas that absorbs harmful UV radiation before it reaches earth, but any concerns about ozone depletion have nothing to do with the greenhouse effect. The problem of ozone depletion is one that has improved significantly due to measures taken to reduce certain types of emissions; humans can get it right sometimes! You do not need to know the specific sources of each type of greenhouse gas – all you need to know is that each greenhouse gas has both natural and man-made origins Page 8 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Greenhouse Gases & Infrared Radiation While only around 25% of the (mostly short wavelength) solar radiation is absorbed by the atmosphere Your notes on its way to Earth, around 80% of the (long wavelength) re-emitted radiation from Earth is absorbed on its way back into the atmosphere For example, incoming UV radiation is absorbed by ozone Re-emitted infrared radiation is absorbed by the main greenhouse gases This absorbed radiation keeps Earth at a habitable temperature However, if there is an imbalance in the chemical composition of the atmosphere, this can lead to fluctuations in the Earth’s mean surface temperature The relative significance of a greenhouse gas depends on its concentration in the Earth's atmosphere and how much the gas can absorb specific wavelengths of radiation Page 9 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The degree of absorption for a particular molecule varies depending on the wavelength of the radiation. The dark parts show the percentage of radiation that is absorbed by each type of greenhouse gas. Ozone (O3) Ozone absorbs close to 100% of the Sun's incoming ultraviolet rays Page 10 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources It also strongly absorbs the wavelengths of the outgoing infrared radiation leaving the Earth's atmosphere, between 9 μm and 10 μm However, it is not a significant contributor to the greenhouse effect as it is found in much smaller Your notes concentrations in the atmosphere Carbon dioxide (CO2) Carbon dioxide is a good absorber of infrared radiation with wavelengths between 1.5 - 30 µm In particular, it strongly absorbs radiation with a wavelength of 15 μm The increasing concentration of carbon dioxide in the atmosphere makes it one of the most significant contributors to the greenhouse effect Water vapour (H2O) Water vapour is the best absorber of infrared radiation with wavelengths between 0.8 - 35 µm The concentration of water vapour in the atmosphere increases as the air becomes warmer Total atmosphere Overall, most of the ultraviolet, infrared and microwave radiation is absorbed by the atmosphere The atmosphere is mostly transparent to incoming visible radiation, which means that the gases in the atmosphere do not absorb or emit much visible radiation Page 11 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Greenhouse Effect Your notes The Greenhouse Effect While only around 25% of the (primarily short wavelength) solar radiation is absorbed by the atmosphere on its way to Earth, around 80% of the (long wavelength) re-emitted radiation from Earth is absorbed on its way back into the atmosphere For example, incoming UV radiation is absorbed by ozone Re-emitted infrared radiation is absorbed by greenhouse gases This absorbed radiation keeps Earth at a habitable temperature However, if there is an imbalance in the chemical composition of the atmosphere, this can lead to fluctuations in the Earth’s mean surface temperature Resonance Model of Global Warming Incoming radiation from the Sun predominantly takes the form of ultraviolet and visible radiation Visible light is not absorbed by the atmosphere, instead, it is absorbed by the Earth’s surface At night, the Earth re-radiates this radiation as infrared Some of this radiation is absorbed by the Earth’s atmosphere and some of the radiation is reflected back into space The greenhouse gases present in the atmosphere absorb infrared radiation and reflect it back towards the Earth’s surface The higher the concentration of greenhouse gases present, the more infrared radiation there is remaining in the Earth’s surface-atmosphere system Therefore, heat energy becomes trapped inside Earth’s atmosphere and accumulates This leads to the greenhouse effect and an increase in average mean temperatures on Earth Page 12 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Greenhouse gases absorb the long-wave radiation emitted by Earth, warming the atmosphere Molecular Energy Level Model The greenhouse effect occurs due to the particular molecular structure of greenhouse gases High-frequency UV light is energetic and able to break bonds within molecules Infrared light, on the other hand, causes atoms to vibrate The greenhouse gases have a natural frequency that falls in the infrared region This means when they absorb infrared light, they begin to resonate, causing the molecules to heat up They absorb the infrared radiation and subsequently emit it back towards the Earth’s surface Page 13 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Solar radiation is primarily short-wave, while the radiation that is re-emitted by earth is long-wave radiation Page 14 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Enhanced Greenhouse Effect Human activity is increasing the number of greenhouse gases in the atmosphere: Your notes Carbon dioxide (CO2) levels in the atmosphere have increased by more than 100 parts per million (ppm) to 420ppm in 2020 Increased amounts of greenhouse gases have led to the enhanced greenhouse effect: Less long-wave radiation (heat) can escape the atmosphere Average global temperatures have increased over 1°C since pre-industrial times The average global temperature has risen remarkably in the past 100 years Page 15 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The greenhouse effect arises due to natural causes, but the enhanced greenhouse effect arises due to human activity Human Sources of Greenhouse Gases Greenhouse Sources from human activity Gas Carbon Dioxide Burning of fossil fuels - power stations, vehicles (CO2) Burning of wood Deforestation - trees utilise CO2 in photosynthesis. The fewer trees there are the less CO2 is removed from the atmosphere Page 16 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Methane (CH4) Decay of organic matter - manure, waste in landfill, crops Nitrous Oxide Artificial fertilisers (N2O) Burning fossil fuels Worked example Which of the following is the result of the enhanced greenhouse effect? A. increasing global average temperature due to natural causes B. decreasing global average temperature due to human activity C. increasing global average temperature due to human activity D. decreasing global average temperature due to natural causes Answer: C The enhanced greenhouse effect causes the average global temperature to increase and is the result of human activity Page 17 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Energy Balance Problems Your notes Energy Balance Problems It is useful to consider Earth’s energy balance in terms of how much incoming energy from the Sun is used and how much is returned to space If incoming and outgoing energy are in balance, the Earth’s temperature will remain constant This can be used to create models which can help climate scientists predict temperature fluctuations based on current and increased concentrations of greenhouse gases At it’s simplest, the model involves a one-layer atmosphere above the Earth’s surface Page 18 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes The diagram below shows a simple energy balance climate model in which the atmosphere and the Earth’s surface are treated as two bodies. The Earth’s surface receives both solar radiation and radiation emitted from the atmosphere. At current atmospheric greenhouse gas concentrations, the temperature of Earth’s atmosphere is set to increase by 6 K. Data for this model: Current mean temperature of the Earth’s atmosphere = 242 K Current mean temperature of the Earth’s surface = 288 K Solar intensity per unit area at top of the atmosphere = 344 W m–2 Emissivity of the atmosphere, e = 0.720 Albedo of the atmosphere, a = 0.280 Use this data to estimate the increase in temperature of the Earth’s surface. Answer: Step 1: List the known quantities Solar intensity above atmosphere, Ia = 344 W m–2 Emissivity of the atmosphere, e = 0.720 Emissivity of the surface, e = 1 New temperature of Earth’s atmosphere, Ta = 242 + 6 = 248 K Page 19 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Stefan-Boltzmann constant, σ = 5.67 × 10–8 W m–2 K–4 Intensity absorbed at the Earth’s surface = Is New temperature of Earth’s surface = Ts Your notes Step 2: Calculate the solar intensity absorbed at the Earth’s surface This can be calculated using the emissivity and the solar intensity above the atmosphere Is = e × Ia Is = 0.720 × 344 = 247.68 = 248 W m–2 Step 3: Write the equation for the power per unit area emitted by a body Since intensity = power per unit area I = e σ T4 Step 4: Calculate the new intensity radiated by the atmosphere I = 0.720 × (5.67 × 10–8) × 2484 = 154.43 = 154 W m–2 Step 5: Calculate the new intensity absorbed by the Earth’s surface The intensity absorbed by the Earth’s surface is a sum of the solar radiation that reaches the surface plus the intensity radiated by the atmosphere New intensity, Is = 248 + 154 = 402 W m–2 Step 6: Calculate the new temperature of the Earth’s surface The Earth’s surface can be assumed to be a black body, hence e = 1 I s = σ Ts 4 400 = (5.67 × 10–8) × Ts4 400 Ts = 4 = 290 K 5. 67 × 10 −8 Step 7: Determine the increase in temperature ΔT = 290 – 288 = 2 K Examiner Tip In simplified climate models, you can generally assume the Earth’s surface and the atmosphere: Act as black bodies - this means the emissivity of the surface will be equal to 1! Remain at a constant temperature Page 20 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers